Evaluation method for cleanliness of metal

ABSTRACT

In order to quickly and economically evaluate cleanliness of a metal with high representativity when quantities, compositions, etc., of non-metallic inclusion particles existing in a metal and resulting in product defects are evaluated by a sample collected during the production process of the metal, the present invention provides an evaluation method involving the steps of levitation-melting a metal piece for a predetermined time by cold crucible levitation-melting means, discharging non-metallic inclusion particles contained in the metal piece to the surface of a molten metal, and directly analyzing a curved and non-smooth sample surface after solidification by a fluorescent X-ray analysis method using an energy dispersion type spectroscope, or by other chemical or physical measurements, to measure or analyze the quantities of elements constituting the non-metallic inclusion particles and to determine quantity of the non-metallic inclusions.

TECHNICAL FIELD

In conjunction with non-metallic inclusions contained in a metal, thepresent invention relates to an evaluation apparatus for cleanliness ofa metal, and a method therefor, which quickly discharges non-metallicinclusions contained in a steel, for example, to the surface portion,detects the non-metallic inclusions accumulating the surface eitherchemically or physically, and accurately determines the proportion ofthe non-metallic inclusions in the metal as a total quantity evaluationor as an evaluation of principal components in accordance with aparticle size distribution.

BACKGROUND ART

Hereinafter, the explanation will be given using steel as a typicalexample of a metal. Non-metallic inclusion particles existing in thesteel include alumina type inclusions formed as the result of thereaction between oxygen in the steel and aluminum added in the case ofan aluminum killed steel, slag type inclusions containing lime/silica,etc., and resulting from a steel making slag, powder type inclusionsresulting from a casting mold lubricant in continuous casting, and soforth. Since these inclusions result in defects such as flows andbreakage in intermediate products, during rolling of thin sheets, wirematerials, etc., or in final products, evaluation of these inclusions byvarious methods have bean carried out in the past for the purpose ofquality control.

If any defects are found in the final product, on the other hand, it isa serious problem to discard the product at the final stage from theaspect of the production cost because the product is produced throughvarious production steps. It is therefore desirable to evaluate qualityat an early stage of the production. Particularly because the existenceof the inclusions is determined at the stage of refining/solidificationof the metal, various evaluation technologies have been conducted in thepast.

The evaluation technology of the inclusions of the steel among themetals is described, for example, in "Steel Handbook, 3rd Edition", IIPig Iron & Steel Making (edited by Japan Iron & Steel Institute ofJapan, published by Maruzen, Oct. 15, 1979). Examples of the evaluationmethods include a total oxygen (T[O]) method based on the oxygenconcentration in the steel, a slime method by electrolytic extractionused for evaluating large inclusions, a microscopic method forevaluating the inclusions by magnifying and observing the section of ametal, and so forth. Due to their respective features, thesetechnologies are limited by the kind of inclusions as the investigationobject and the sizes of the inclusions as tabulated in Table 1, and theyare not free from the problem, either, that a long time is necessarydepending on the evaluation method.

It is known that information of intermediate products is not sufficientso as to estimate the product defects. In other words, as shown in Table1, the conventional means involves the problems that the evaluationsample does not sufficiently represent the quality of the intermediateproduct and a long time is necessary for the evaluation of the sample,and those methods which invite excessively great super-heat duringmelting such as an EB (electron beam melting) method involve the problemthat the inclusions are denatured during evaluation.

The slime method has been widely employed as a method having relativelyhigh accuracy, but an extremely long time of several days to dozens ofdays is necessary to electrolyze about 1 kg sample as a whole.

When the evaluation is made by a small amount of metal sample, a metalpiece sample of a part of large amounts of metal is evaluated.Therefore, to strictly evaluate the cleanliness of the whole metal, alarge number of samples must be collected from the same metal piece, andthe problem to be solved is to speed up the evaluation of thecleanliness.

                                      TABLE 1                                     __________________________________________________________________________           particle diameter                                                                      evaluation quantity                                             name of inclusions & necessary time others                                  __________________________________________________________________________    microscope                                                                           up to 40 μm                                                                         100 positions, 25 mm.sup.2                                        several days                                                                T[O] -- --                                                                    slime at least 40 μm several kg, several                                     to dozens of days                                                           EB up to 200 μm 2 g (several pcs) one components evaporate                   day due to reduced pressure                                                 This Invention not limited hundred to thousand components do not                                          grams, about 10 min. evaporate due to Ar                                       atmospheric pressure                           __________________________________________________________________________

On the other hand, though the melting means is different from the EBmethod, an induction melting extraction method using a cold cruciblemethod is conceivable as the same melting extraction method. In otherwords, this method eliminates the problems such as high temperaturemelting of the EB method and the resulting modification of theinclusions, and insufficiency as the representative value by theevaluation volume of the small amount. A method of measuring theinclusions of the surface of the sample produced by this cold cruciblelevitation-melting method is described, for example, in "Evaluation ofAlloy Cleanness in Superclean Materials", K. C. Mills et al., TurkdoganSymposium Proceedings, pp. 105-112 (1994). The method of this referenceinspects the surface inclusions by a scanning electron microscope.However, this reference points out only the problem as the evaluationmethod by the characteristics of the cold crucible itself, but does notteach the method of evaluating the non-metallic inclusions over a widearea of the metal surface industrially, economically and quickly.

FIGS. 1(a) and 1(b) are explanatory views of the principal portions of acold crucible apparatus, wherein FIG. 1(a) is an explanatory plan view,and FIG. 1(b) is an explanatory view of the longitudinal section takenalong A--A of FIG. 1(a). In FIG. 1, reference numerals (1-1, . . . ,1-8) denote eight, for example, copper segments which together form acrucible and the inside of which is cooled with water. They are disposedadjacent to one another with the gap slits 3 interposed at a pluralityof substantially equidistant positions and form the crucible. Referencenumeral 2 in the drawings denotes an induction coil, which is sodisposed as to encompass the crucible.

FIGS. 2(a) and 2(b) are explanatory views of the operation of the coldcrucible. When a high frequency current flows through the induction coil2 in a direction indicated by an arrow 5, an inducted electromotiveforce occurs in a direction indicated by an arrow 6-1 occurs on the sideof the induction coil 2 of the segments 1. Since the segments 1 arespaced apart from one another by the slits 3, however, the inductioncurrent does not flow through other adjacent segments, but flows as aninduction current in a direction indicated by an arrow 6-2 on theopposite side to the induction coil. Reference numeral 4 in the drawingrepresents the metal sample. An eddy current flows through the metalsample 4 in a direction indicated by an arrow 7 due to the inductioncurrent in a direction indicated by an arrow 6-2. The metal piece 4 isheated by the eddy current in the direction of the arrow 7 and ismelted. In this instance, since the eddy current flows through themolten metal 4 in the direction of the arrow 7, repulsion 8 acts in thecenter direction of the metal due to the induction current in thedirection of the arrow 6-2 that flows through the segments 1, and thisrepulsion 8 keeps the molten metal 4, under a levitating and non-contactstate, away from the segments 1.

The cold crucible method melts the metal sample, due to levitation, in anon-oxidizing atmosphere and holds the levitating molten metal. Duringthis retention time, the non-metallic inclusions in the metal sample aredischarged to the surface of the molten metal as indicated by referencenumeral 9 in FIG. 2(b). When the current to be passed through the coilis cut off after retention for a predetermined time, the molten metal issolidified while the non-metallic inclusions gather on the surfacethereof. The cleanliness of the metal piece is evaluated by measuringthe non-metallic inclusions gathered on the surface of the solidifiedbody.

According to the prior art method which measures the non-metallicinclusions scattered inside the metal piece, measurement is complicatedand requires a long measurement time but according to the cold cruciblemethod, the measurement of the non-metallic inclusions gathering on thesurface can be easily made because they gather on the surface of thesolidified body and, moreover, within a short time. According to theprior art method which measures the non-metallic inclusions containedand scattered in the metal, the sample is extremely small, and is notcorrect as a representative value of the steel. On the other hand,because the cold crucible method can levitate several grams to severalkilograms of the metal sample, the quantity of the sample is greaterthan before, and evaluation can be made more correctly over a typicalvalues of the steel.

SUMMARY OF THE INVENTION

If the quality of intermediate products corresponding to quality ofproducts of metal pieces can be quickly evaluated as compared to theprior art, the production cost and time can be drastically improved. Thepresent invention is completed on the basis of this concept.

In other words, the present invention is directed to solve the problemsof representativity of the evaluation samples in quality evaluation ofthe intermediate products, the problems of the measurement time andcost, and the problem of denaturing of inclusions. If the cold crucibletreatment alone is merely carried out and the non-metallic inclusionsare merely gathered on the sample surface, it takes a long time toinvestigate the surface by using the microscope and to count the numberof the non-metallic inclusions, as described in the reference describedabove, and the intended objects cannot be accomplished.

To accomplish the objects described above, the present inventionprovides an apparatus, and a method therefor, which can gathernon-metallic inclusions to the most advantageous position for themeasurement of the whole quantity by a cold crucible, and canefficiently measure the whole quantity.

The gist of the present invention resides in the following points.

(1) An evaluation apparatus for cleanliness of a metal, comprising:metal levitation-melting means which comprises a water-cooled metalcrucible including a bottom surface having a curvature and a sidewallsurface having a sloped surface gradually expanding upward, and havingslits interposed in a radial direction, an induction coil for generatinga repulsion from the sidewall surface of the water-cooled metal crucibleto a center direction, and passing a high frequency current for meltingthe metal while levitating the metal, and a container for maintaining anon-oxidizing atmosphere; handling means for taking out a metal havingnon-metallic inclusions accumulating at a specific position on thesurface of the metal melted and solidified inside the metallevitation-melting means, and transferring the metal to analyzing means;and the analyzing means for analyzing the non-metallic inclusions soaccumulated.

(2) An evaluation apparatus for cleanliness of a metal according to theitem (1), comprising: metal levitation-melting means which comprises awater-cooled metal crucible comprising a plurality of segments dividedin a circumferential direction, and having an open upper surface and aclosed lower surface, an induction coil for passing a high frequencycurrent, disposed in such a manner as to encompass the water-cooledmetal crucible, and a non-oxidizing atmosphere container; handling meansfor taking out a metal melted and solidified by the levitation-meltingmeans from the water-cooled metal crucible, moving the metal, andcapable of setting the metal to a predetermined analysis position; andenergy dispersion type fluorescent X-ray means for analyzingnon-metallic inclusions accumulating on the surface of the metal.

(3) An evaluation apparatus for cleanliness of a metal according to theitem (1), comprising: metal levitation-melting means which comprises awater-cooled metal crucible comprising a plurality of segments dividedin a circumferential direction, and having an open upper surface and aclosed lower surface, an induction coil for passing a high frequencycurrent, disposed in such a manner as to encompass the water-cooledmetal crucible, and a non-oxidizing atmosphere container; metaltransferring means for taking out a metal melted and solidified by thelevitation-melting means from the water-cooled metal crucible, andtransferring the metal to predetermined processing means; andacid-dissolving or electrolyzing means for extracting non-metallicinclusions concentrated on the surface of the metal melted andsolidified by the processing means.

(4) An evaluation apparatus for cleanliness of a metal according to theitem (1), comprising: metal-levitation means which comprises awater-cooled metal crucible comprising a plurality of segments dividedin a circumferential direction, and having an open upper surface and aclosed lower surface, an induction coil for passing a high frequencythree-phase alternating current for imparting a repulsion moving upwardon the surface of a molten metal along the wall of the crucible whilelevitating and melting the metal thereinside, disposed in such a manneras to encompass the water-cooled metal crucible; and luminancedifference/area conversion means for analyzing non-metallic inclusionsaccumulating on the upper surface of the metal melted and solidified bythe levitation-melting means.

(5) An evaluation apparatus for cleanliness of a metal according to theitem (1), wherein means for supplying a current to be passed through theinduction coil is a single-phase alternating current source.

(6) An evaluation apparatus for cleanliness of a metal according to anyof the items (2) through (4), wherein the shape of the inner surface ofthe crucible has a shape formed by cutting a rotating body having thesymmetry axis of a perpendicular axis into halves on a plane ofsymmetry, and a shape formed by an upper shape of a circular truncatedcone having the same shape as that of the symmetry plane or an upwardlyexpanded similar shape of the horizontal section.

(7) An evaluation apparatus for cleanliness of a metal according to anyof the items (2) through (4), wherein the bottom surface of the crucibleis shaped in such a manner that the bottom of the inner surface in anarea of at least 90% by the diameter of the inner surface becomes a flatsurface.

(8) An evaluation method for cleanliness of a metal comprising the stepsof: levitation-melting a metal piece for a predetermined time by usinglevitation-melting means; discharging non-metallic inclusions containedin the metal piece to the surface of a molten metal; and directlyanalyzing a curved and non-smooth surface of the metal aftersolidification by a fluorescent X-ray analysis means using an energydispersion type spectroscope so as to measure quantities of elementsconstituting the non-metallic inclusions and to identify the quantity ofthe non-metallic inclusions.

(9) An evaluation method for cleanliness of a metal according to theitem (8), comprising the steps of: levitation-melting a metal piece fora predetermined time by using levitation-melting means; dischargingnon-metallic inclusions contained in the metal piece to the surface of amolten metal; rotating either intermittently or continuously the metalhaving a curved and non-smooth surface round an axis connecting theuppermost point and the lowermost point at the time of melting as thecenter thereof; directly analyzing the surface of the metal by afluorescent X-ray analysis means using an energy dispersion typespectroscope; measuring the quantities of elements constituting thenon-metallic inclusions; and identifying the quantities of thenon-metallic inclusions in accordance with the kind or the origin.

(10) An evaluation method for cleanliness of a metal comprising thesteps of: levitation-melting a metal piece for a predetermined time byusing levitation-melting means; discharging non-metallic inclusionscontained in the metal piece to the surface of a molten metal;dissolving the surface of the metal after solidification by an acidicsolution or electrolyzing it in an aqueous type solution or anon-aqueous type solution; extracting and filtrating the non-metallicinclusions; and weighing and analyzing the non-metallic inclusions sofiltrated, or weighing and analyzing them after separation.

(11) An evaluation method for cleanliness of a metal according to theitem (10), wherein the retention time t (seconds) of thelevitation-melted metal for accumulating the non-metallic inclusionscontained in the metal to the surface of the levitation-melted metalfalls within the following range (1): ##EQU1## where d is a maximuminner diameter (mm) of the crucible.

(12) An evaluation method for cleanliness of a metal characterized inthat measurement of non-metallic inclusions accumulating on the surfaceof the top of a molten metal is carried out by the steps of cutting offa high frequency current after a metal sample is levitation-melted, thedifference of luminance between the surface of the metal sample duringcooling and the non-metallic inclusions is photographed by a CCD camera,and island-like occupying areas of the non-metallic inclusions aremeasured by image processing the image so photographed.

(13) An evaluation method for cleanliness of a metal according to theitem (11), comprising the steps of: carrying out a levitation-meltingtreatment by changing t/√ (30 d) (t: retention time of alevitation-melted metal (seconds), d: maximum inner diameter (mm) ofcrucible); determining in advance the relation between t/√ (30 d) and adiameter L of the non-metallic inclusions by investigating the diameterL occurring at maximum frequency at each t/√ (30 d) value; selecting adesired value for t/√ (30 d) when the cleanliness of another metal isevaluated, and carrying out the levitation-melting treatment for theother metal; measuring the occurring quantity N of the non-metallicinclusions having the diameter L in the other metal by estimating thatthe diameter L of the non-metallic inclusions occurring at the maximumfrequency in the other metal at this selected t/√ (30 d) value is thesame as the relation that is determined in advance; and evaluating thisN as cleanliness of the other metal.

(14) An evaluation method for cleanliness of a metal according to theitem (13), wherein the occurring quantities N₁, N₂, . . . of thenon-metallic inclusions having diameters L₁, L₂, . . . greater than Lare measured in the other metal, and the N₁, L₂, . . . values areevaluated as cleanliness of the other metal.

(15) An evaluation method for cleanliness of a metal according to theitem (10), wherein at least 10 particles are selected from particleshaving the maximum diameter in the non-metallic inclusions discharged,and the diameters of the non-metallic inclusions having the maximumparticle diameters existing in the metal from which the metal piece iscollected, are estimated by a statistical extremes method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is an explanatory view of principal portions of a coldcrucible apparatus.

FIG. 1(b) is a longitudinal section view taken along a line A--A of FIG.1(a).

FIG. 2(a) is an explanatory view of the operation of the cold crucible.

FIG. 2(b) is a longitudinal sectional view of FIG. 2(a).

FIG. 3 is an explanatory view showing the crucible shape of the coldcrucible apparatus.

FIG. 4 is an explanatory view showing the flows of a high frequencycurrent and an eddy current.

FIG. 5 shows the relation between a levitation-melting retention timeand a non-metallic inclusion discharge ratio.

FIG. 6 shows an existence ratio of levitation non-metallic inclusionsdepending on the surface depth.

FIG. 7(a) is a view useful for explaining movement of non-metallicinclusions accumulating on the surface of a levitation-melted metalduring steady levitation melting.

FIG. 7(b) is a view showing the positions of non-metallic inclusions onthe surface when the supply of power to a coil is stopped.

FIG. 8 is a view showing a three-phase A.C. cold crucible apparatus.

FIG. 9 is a view showing the relation between an electromagnetic forceacting on a molten metal and surface tension, etc.

FIG. 10 is a diagram showing non-metallic inclusion distributions of abase metal and a levitation-melted material by a surface electrolyticmethod.

FIG. 11 is a view showing a sample collection position oflevitation-melting.

FIG. 12 is a view showing an example of the size of a crucible forlevitation-melting used for an embodiment.

FIG. 13(a) is a diagram showing the correlation between an aluminaanalysis result and a total oxygen concentration of Example 1.

FIG. 13(b) is a diagram showing the relation between product defects anda non-metallic inclusion index.

FIG. 13(c) is a diagram showing the correlation between an aluminaanalysis result and a total oxygen concentration in Example 3.

FIG. 14 is a diagram showing the correlation between a CaO analysisresult and an analysis result by a slime method.

FIG. 15 is a diagram showing the relation between the number ofextracted non-metallic inclusions of a cold crucible melted material andthe number of extracted inclusions of a slime method according to theprior art.

FIG. 16 is a diagram showing the relation of a grain size ofnon-metallic inclusions and the proportion of the number of non-metallicinclusions.

FIG. 17 is a diagram showing an evaluation example of non-metallicinclusions in an iron sample.

FIG. 18 is a diagram showing the occurrence state of non-metallicinclusion particles by a cold crucible having a maximum inner cruciblediameter of 30 mm.

FIG. 19 is a diagram showing the occurrent state of non-metallicinclusion particles by a cold crucible having a maximum inner cruciblediameter of 100 mm.

FIG. 20 is a diagram showing the occurrence state of non-metallicinclusion particles in a continuous casting slab different from that ofFIG. 18.

FIG. 21 is a diagram showing the relation between an occupation ratio ofisland-like non-metallic inclusions after solidification shown in Table3 and the quantity of non-metallic inclusions.

FIG. 22 is a diagram showing the relation between an occupation ratio ofisland-like non-metallic inclusions at 15 seconds from cut-off of acurrent solidified by reducing a current of 80% of a reference currentand shown in Table 3 and the quantity of non-metallic inclusions.

FIG. 23 is a diagram showing the relation between an occupation ratio ofisland-like non-metallic inclusions at 15 second from cut-off of acurrent solidified by reducing a current to 90% of a reference currentand shown in Table 3 and the quantity of non-metallic inclusions.

FIG. 24 is a diagram showing the relation between an occupation ratio ofnon-metallic inclusions by a surface electrolysis method and thequantity of non-metallic inclusions.

BEST MODE FOR CARRYING OUT THE INVENTION

To efficiently detect the whole quantity of non-metallic inclusions of asample, the present invention carries out levitation-melting so that theaggregate of non-metallic inclusions to be discharged can be controlledto an optimum position of the sample. In this way, the discharge andaggregate position can be set easily and quickly to the position atwhich analysis by an energy dispersion type fluorescent X-ray apparatuscan be executed. When the non-metallic inclusions can be aggregated nearthe center of the upper surface of the sample surface portion, the setposition can be positioned to the X-ray visual field. When they areaggregated to the center of the side surface, the non-metallicinclusions so aggregated can be efficiently analyzed by rotating theX-ray source. The characterizing feature of the present inventionresides in that cleanliness of metals can be evaluated economically andquickly with high reproducibility.

Hereinafter, a concrete construction of the method of the presentinvention will be described with reference to FIG. 3.

A metal crucible (cold crucible) 13 comprising metal segments 1 dividedin a circumferential direction and having an open upper surface and aclosed lower surface as shown in FIG. 2(a) is disposed inside acontainer 11 capable of controlling an inert gas atmosphere or a vacuumatmosphere 10 as a non-oxidizing atmosphere. The shape of this cruciblemay be such that its bottom surface has a curvature and its sidewallsurface has an inclination so that the inner diameter progressivelyincreases towards the upper portion. The crucible is encompassed by awater cooled coil 16 through which a high frequency current 15 given bya high frequency transmitter 14 is caused to flow. A metal sample 6 asan object whose weight is measured in advance is placed inside thecrucible and then the current is caused to flow through the coil. Then,the molten metal 6 levitates inside the water cooled metal crucible 13due to the resulting electromagnetic force 17. FIG. 4 is an explanatoryview of the flows of the high frequency current and an induction currentat this time. When the high frequency current flows through theinduction coil, induction electromotive force 5 develops on theinduction coil side of the segments. Since the segments are mutuallyseparated by the slits, however, the induction current does not flowthrough the adjacent segments but flows as the induction current on theopposite side of the high frequency coil of the segments. An eddycurrent 7 flows through the metal sample due to the induction current.The metal piece is heated and melted by this eddy current 7. In thisinstance, because the eddy current flows through the molten metal,repulsion 8 operates in the center direction of the metal due to theinduction current flowing through the segment, and this repulsion 8keeps the molten metal in the levitated state while it is out of contactwith the segments. Because the sectional area of the inside of thecrucible progressively decreases towards the lower portion, a strongerelectromagnetic force acts on the metal at a lower portion. Inconsequence, the balance of the electromagnetic force 17, surfacetension 19 and gravity 20 is established at the time of melting as shownin FIG. 9, and the molten metal levitates inside the crucible. Thespecific gravities of the non-metallic inclusions are smaller than thatof the molten metal, the reaction to the push force of the inducedelectromagnetic force that pushes inward the molten metal acts on thenon-metallic inclusions and furthermore, surface tension exists betweenthe non-metallic inclusions and the molten metal. Therefore, thelevitation melted body is discharged to the outer periphery 18. Afterretention for a predetermined time, the electric current through thecoil is cut off. Then, the molten metal is solidified, and thenon-metallic inclusions accumulate on the surface of the levitationmelted body.

The present invention comprises an invention for accumulating thenon-metallic inclusions discharged to the surface to a position at whichevaluation can be quickly made, and an invention for quickly determiningthe quantity, composition and grain size distribution of thenon-metallic inclusions existing on the non-smooth surface aftersolidification.

The method of evaluating the quantity of the discharged non-metallicinclusions as a characterizing feature of the present invention is amethod of evaluating cleanliness of a metal which comprises the steps oflevitation-melting a metal piece for a predetermined time by a coldcrucible levitation-melting apparatus, discharging non-metallicinclusions existing inside the metal piece, accumulating thenon-metallic inclusions discharged to the surface, directly analyzingthe same surface after levitation-melting and solidification by afluorescent X-ray analysis method using an energy dispersion typespectroscope, measuring the quantities of elements constituting thenon-metallic inclusions, and determining the quantity of thenon-metallic inclusions.

The surface of the sample levitation-melted and solidified by the coldcrucible levitation-melting apparatus is curved and non-smooth. Further,the non-metallic inclusions discharged to the sample surface exists inthe island form and non-uniformly on the sample surface. To quickly andeasily analyze the non-metallic inclusions existing under such a state,the present invention uses the energy dispersion type spectroscope forthe fluorescent X-ray analyzer, and measures a relatively broad region(several mmφ and preferably, at least 10 mmφ). The non-metallicinclusions can be analyzed in further detail and more precisely byanalyzing the entire surface of the solidified sample. The presentinvention can display typically the quantity of the non-metallicinclusions of the whole metal by analyzing the non-metallic inclusionson the metal surface.

The method of analyzing the quantity, composition and grain sizedistribution of the discharged non-metallic inclusions as anothercharacterizing feature of the present invention is a method ofevaluating cleanliness of a metal which comprises the steps oflevitation-melting a metal piece for a predetermined time by a coldcrucible levitation-melting apparatus, discharging non-metallicinclusions existing inside the metal piece to the surface of a moltenbody, accumulating the non-metallic inclusions discharged to thesurface, then electrolyzing the surface of the sample afterlevitation-melting and solidification in an acid solution or ahalogen/alcohol solution (e.g. bromomethanol solution), or an aqueoustype solution (e.g. 10% ferric chloride solution, sodium citratesolution), or a non-aqueous type solution (e.g. acetylacetone solution),extracting and filtrating the non-metallic inclusions, and weighing andanalyzing, or weighing and analyzing after separation according to thegrain size, the non-metallic inclusions so filtrated.

Further, the present invention carries out a cold crucible treatment bychanging t/√ (30 d) (t: retention time for levitation-melting (second),d: maximum inner diameter of the crucible (mm)), examines the diameter Lof the non-metallic inclusions occurring at the maximum frequency ineach t/√ (30 d), and determines in advance the relation between t/√ (30d) and L (diameter of impurity particles). Next, when cleanliness ofanother metal is evaluated, a desired value is selected for t/√ (30 d),and the cold crucible treatment of the other metal is carried out. Theoccurrence quantity of N of non-metallic inclusions having the diameterL in the other metal is measured by assuming that the diameter L of thenon-metallic inclusions occurring at the maximum frequency in the othermetal in this selected t/√ (30 d) is the same as that of the metaldescribed above, and N is evaluated as cleanliness of the other metal.The present invention provides also an evaluation method of cleanlinessof a metal characterized in that the generation quantities N₁, N₂, . . .of non-metallic inclusions having diameters of L₁, L₂, . . . greaterthan L in the other metal are measured, and these N₁, N₂, . . . valuesare evaluated as cleanliness of the other metal.

The metal surface after solidification is observed under magnificationusing a microscope, etc., and the number of non-metallic inclusionshaving statistical meaning, that is, at least 10 and preferably at least40, of the non-metallic inclusions, are selected from those having themaximum diameter, and are plotted on an extreme value statistical chart.The particles having the maximum particle size are then estimated. Thisextreme value statistical chart is described in detail, for example, inGumbel "Statistics of Extremes" (published by Seisan-Gijutsu CenterShinsha, Jun. 15, 1978). The outline of this means is as follows in thecase of the method of the present invention. A metal is melted, andnon-metallic inclusions extracted are then magnified and photographed bya microscope. At least 10, and preferably at least 40, non-metallicinclusions inside the visual field are measured. The number ofnon-metallic inclusions so measured are re-arranged serially from thesmaller size and a cumulative distribution function value is calculatedand are plotted on the extreme value probability chart. Next, arecursive formula is calculated, and the maximum non-metallic inclusionsare estimated. According to the method of the present invention,cleanness of a metal can be evaluated economically, quickly and withhigh representativity.

The crucible has the afore-mentioned shape. For example, it is a knowncrucible ("Material Processing Utilizing Electromagnetic Force". Nos.129 and 130th Nishiyama Memorial Technical Lectures, published by Iron &Steel Institute of Japan, Foundation, Apr. 28, 1989) which is called a"batch type crucible" or a "levitation-melting type crucible", whoseupper surface is opened and whose lower surface is closed.

FIG. 5 is a diagram showing the relation between an alumina dischargeratio in the sample and a levitation-melting retention time. A sample ofa weight of dozens of grams to several kilo-grams is levitation-meltedby the cold crucible levitation-melting method, and according to theresult of the experiments conducted by the present inventors by using ametal piece of 100 g, it can be seen from FIG. 5 that about 80% of thenon-metallic inclusions in the sample are discharged when melting isretained for at least 3 minutes and the discharge ratio does not altereven when the melting retention time is kept longer than 3 minutes. Theresult of the experiments conducted by the present inventors revealsthat almost all the non-metallic inclusions and impurities that aredischarged to the surface exist within the depth of about 30 μm from thesurface layer (see FIG. 6). On the other hand, X-ray transmittance isabout 100 μm for iron and dozens of μm for alumina as a typicalnon-metallic inclusion. Therefore, in order to directly measure only theregion in which the non-metallic inclusions discharged exist, it is mostefficient to apply the fluorescent X-ray analysis as is used in thepresent invention.

The surface of the sample again solidified after levitation-melting is acurved surface and is a non-smooth surface. Therefore, the sample cannotbe measured by a wavelength dispersion type fluorescent X-ray methodwhich is generally employed for elementary analysis. For this reason,the present invention uses the energy dispersion type fluorescent X-rayanalysis method capable of measuring the curved and non-smooth surfaceat the sacrifice of analytical accuracy to some extent.

As described above, the non-metallic inclusions discharged to the samplesurface exist in the island form and under an extremely heterogeneousstate. The experiment conducted by the present inventors teaches that inorder to analyze the non-metallic inclusions under such a state, it isnecessary to measure at once the regions of several millimeters or tomeasure several small regions. As a matter of fact, the measurementresult having a high level of accuracy can be obtained when measurementis conducted by setting the primary X-ray beams to at least 10 mm. Ifpossible, it is desired to irradiate the primary X-rays to the wholesurface of the sample.

On the other hand, most of the fluorescent X-ray analyzers commerciallyavailable at present employ predominantly the methods which use narrowprimary X-ray beams and measure a very small region but very fewanalyzers employ the method which expand the primary X-ray beams toseveral millimeters as is used in the present invention.

Because the present invention analyzes the elementary compositions ofthe non-metallic inclusions, and the impurity particles, alumina,calcia, silica, magnesia, sodium oxides, etcs., contained in them can bequickly identified and determined quantitatively in accordance with therespective compositions.

To quickly convey the non-metallic inclusion sample accumulating on thesample surface to the fluorescent X-ray analyzers, etc., the presentinvention disposes an electromagnet or a sucking disk as means fortaking out the sample from the crucible so as to suck the sample and toconvey it to the analyzer disposed in the proximity of the crucible. Theapparatus of the invention includes a handling device having positionsetting means for positioning the accumulating position of thenon-metallic inclusions to a position within the irradiation range ofthe analyzing X-rays.

As described above, the present invention carries out cold cruciblelevitation-melting, analyzes the non-metallic inclusions discharged tothe sample surface by the energy dispersion type fluorescent X-rayanalysis method, and can measure the quantity of the elementsconstituting the non-metallic inclusions to analyze the component or toidentify the components.

As described above, further, almost all the non-metallic inclusionsdischarged to the surface exist within the depth of about 30 μm from thesurface layer. Therefore, the present invention can recover and analyzealmost all the object non-metallic inclusions within an extremely shorttime of several to dozens of minutes, which is by far shorter than theconventional methods, by melting or electrolyzing about 50 to about 100μm of the uppermost surface layer of the sample after solidification. Itcan be appreciated from this fact, too, that the present invention canprovide a method of analyzing the non-metallic inclusions which hassufficient speed and convenience to be used as a management index of thesteel production operation.

When the recovered non-metallic inclusions are isolated in accordancewith the particle size and are then analyzed, not only the componentanalysis of the non-metallic inclusions but also the measurement of theparticle size and the particle size distribution, the component analysisin accordance with the particle size and the composition analysis inaccordance with the particle size become possible.

FIGS. 7(a) and (b) are schematic views useful for explaining themovement of the non-metallic inclusions discharged to the surface of thelevitation-melted metal in the conventional cold crucible method whichapplies an ordinary single-phase radio frequency current to an inductioncoil. FIG. 7(a) in an explanatory view when the current is applied andFIG. 7(b) is an explanatory view when the high frequency current is cutoff. A gentle stream 10 of the melted metal which rises at the centerand flows down along the surface is formed in the levitation-meltedmetal. A part 9-1 of the non-metallic inclusions discharged to thesurface of the molten metal is pushed by this stream 10 of the moltenmetal and moves to the gap between the levitation-melted metal 4 and thesegments 1. When the high frequency current is cut off, the non-metallicinclusions 9-1 do not accumulate on the surface of the top of the moltenmetal but are pushed to the portion in the proximity of the lowersurface of the molten metal as shown in FIG. 7(b). Therefore, whencleanliness of the metal is evaluated in the case of FIG. 7(b), themetal surface of the sample 4 in FIG. 7(b) after solidification ismeasured. However, because the non-metallic inclusions scatter on thesurface and have a broad measurement area, convenience and quickness ofthe evaluation of non-metallic inclusions are not yet sufficient.

Therefore, in the case of levitation-melting using the single-phasealternating current, the non-metallic inclusions according to thepresent method spout up around the axis of symmetry of the molten metaland are deposited between the sidewall and the molten metal while beingcarried by the stream that flows down along the wall of the crucible asshown in FIG. 7(a). When the current to the coil is cut off in thisinstance, the levitating metal is pushed to the bottom of the crucibledue to the gravity as shown in FIG. 7(b), and some of the non-metallicinclusions discharged to the surface are collected by the side portionsof the metal while another part moves on the metal. Here, when thecurrent is once held at a level at which the metal under levitating issolidified, and is then cut off after solidification of the metal, thenon-metallic inclusions are collected only at the side portion and forma band-like accumulating band.

FIG. 8 shows an induction heating coil for supplying high frequencycurrents of U, V and W of the three-phase alternating current havingmutually different phases as the induction heating coil of the presentinvention. This induction heating coil is so constituted as to possessthe function of a linear motor for forming an upward stream 11 on thesurface of the molten metal 4 which is levitation-melted by thethree-phase AC Currents, U, V and W. The high frequency currents U, Vand W are so arranged as to allow the metal sample 4 to belevitation-melted. In other words, the induction heating coil accordingto the present invention levitation-melts the metal sample 4 and formsthe upward stream 11 on the surface of the molten metal which is solevitation-melted. When the three-phase alternating current is used inthe present invention, the stream becomes upward along the wall of thecrucible during melting, too, as shown in FIG. 8. Therefore, thenon-metallic inclusions accumulate only at the upper portion, andaccumulate at the upper portion, even after solidification, irrespectiveof the cut-off operation of the current. Consequently, substantially allof the non-metallic inclusions contained in the metal sample can bedetermined by measuring the non-metallic inclusions of the island-likeoccupation area at the top of the molten metal, and cleanliness of themetal can be evaluated extremely conveniently and quickly.

EXAMPLES Example 1

Twenty cast slabs of low carbon aluminum killed steels were first castby using a casting mold having a width of 1,500 mm and a thickness of250 mm at a casting rate of 1.2 m/min. Samples were collected at 1/4 and1/2 portions from a size of 20 mm in the casting direction, 30 mm fromthe surface layer in the thickness direction and 20 mm in the transversedirection from these slabs, respectively. Each sample was melted in acrucible having an inner diameter of 40 mm, a depth of 40 mm and aparabolic sectional shape within the range of 20 mm to 40 mm from theupper end shown in FIG. 12 in an atmosphere having a gauge pressure of0.2 atms with respect to the atmospheric pressure. A power of 30 kW wasapplied to the coil, and the metal was retained for 5 minutes aftermelting. Then, power was reduced proportionally to 0 kW in the course of10 seconds. The molten sample was solidified under the state devoid ofthe sink and cavity at its top as the final solidification position.Thereafter, the area of the island accumulation of the non-metallicinclusions was examined. Another sample collected from the position veryclose to the collecting position of each sample and having the same sizewas subjected to the total oxygen (T[O]) analysis for the purpose ofcomparison. FIG. 13(a) shows the result of their indices and thisdiagram shows a very close correlationship. Similarly, FIG. 13(b) showsthe index comparison with a defect index of a product sheet afterrolling and surface treatment of the same slab, and a closecorrelationship could be obtained.

Example 2

Slabs of a high carbon steel were first cast by a 160 mm-square castingmold at a casting rate of 2 m/min, and each sample was collected at a1/2 portion of the side of each slab from a size of 20 mm in the castingdirection, 30 mm from the surface layer in the thickness direction and20 mm in a peripheral direction as shown in FIG. 11. Each sample wasmelted in a crucible having an inner diameter of 40 mm, a depth of 40 mmand a parabolic sectional shape within the range of 20 mm to 40 mm fromthe upper end as shown in FIG. 12 in an atmosphere having a gaugepressure of 0.2 atm with respect to the atmospheric pressure. A power of30 kW was applied to the coil, and the molten metal was retained for 5minutes after melting. Power was thereafter reduced. The molten samplewas solidified under the state devoid of sink and cavity at the topthereof as the final solidification position. Thereafter, thenon-metallic inclusions electrolytically extracted from the surface ofthe sample molten and solidified were gathered on a filter, and wereobserved through a microscope. Statistical calculation of extremes ofthe maximum non-metallic inclusions was carried out for each field from50 fields (one field: 0.02833 mm²), and the non-metallic inclusionshaving the maximum particle diameter were estimated. On the other hand,50 samples having the same size were melted, the non-metallic inclusionshaving the maximum particle diameter on each sample surface wereexamined, and they were compared with the estimation result. Table 2shows the estimated particle diameters by the statistical extremes andthe particle diameters of the maximum non-metallic inclusions on thesurface of the fifty samples. A result substantially coincide with theestimated particle diameters could be obtained.

                  TABLE 2                                                         ______________________________________                                                  statistical                                                                              experiment                                                 extremes estimation evaluation value                                        ______________________________________                                        diameter    15 μm     16 μm                                               of maximum                                                                    inclusions                                                                  ______________________________________                                    

Example 3

Twenty cast slabs of a low carbon aluminum killed steel were first castby using a casting mold having a width of 1,500 mm and a thickness of250 mm at a casting rate of 1.2 m/min, and samples were collected at 1/4and 1/2 portions in the transverse direction of the slabs from a size of20 mm in the casing direction, 30 mm from the surface layer in thethickness direction and 20 mm in the transverse direction. Each samplewas melted in a crucible having an inner diameter of 40 mm, a depth of40 mm and a parabolic sectional shape within the range of 20 mm to 40 mmfrom the upper end as shown in FIG. 12 in an Ar atmosphere atatmospheric pressure. The sample was held for 5 minutes after melting,and was solidified after discharging the non-metallic inclusions.

The non-metallic inclusions accumulating in an island form on thesurface of the sample after re-solidification were analyzed byfluorescent X-ray analysis. Measurement was carried out at an intensityof primary X-rays of 1 μA×50 kV, an irradiation diameter of 13 mm and anirradiation time of 30 seconds. The quantity of alumina, silica, calcia,etc., in the sample was measured from the fluorescent X-ray intensity ofAt, Si, Ca, etc. At the same time, each sample collected from theposition closest to the same sample and having the same size wassubjected to the total oxygen analysis. When both measurement resultswere compared, the aluminum intensity obtained by the fluorescent X-rayanalysis and the total oxygen exhibited a close correlation as shown inFIG. 13(c). When calcium in the non-metallic inclusions of a similarsample collected separately was analyzed by the X-ray analysis inaccordance with the method of the present invention and was comparedwith CaO obtained by the slime method to determine the correlationshipwith CaO, they exhibited a close correlation as shown in FIG. 14. It canbe seen from FIG. 16 that the result hardly changes from the result ofthe slime method in the case of the size distribution of a 50 μminterval. FIG. 15 shows the result obtained by measuring the quantity ofthe non-metallic inclusions contained in the sample inside a tundish bythe method of the present invention and comparing it with the slimemethod. As can be seen from the diagram, the quantity of thenon-metallic inclusions was great inside the tundish but was small inthe slab. In this way, the inclusions inside the steel sample could beevaluated economically, quickly and conveniently.

In other words, the method of the present invention can evaluate thenon-metallic inclusions within a time of about 5 minutes for coldcrucible levitation-melting, about 1 minute for the fluorescent X-rayanalysis and a few minutes for fitting the sample to the apparatus, orwithin about 10 minutes in total, and can evaluate quality of the slabwithin a far shorter time than the conventional evaluation method.

Example 4

Twenty slabs of a low carbon aluminum killed steel were first cast byusing a casting mold having a width of 1,500 mm and a thickness of 250mm at a casting rate of 1.2 m/min, and each sample was collected at 1/4and 1/2 portions in the transverse direction of the slabs from a size of20 mm in the casting direction, 30 mm from the surface layer in thethickness direction and 20 mm in the transverse direction. Each samplewas melted in a crucible having an inner diameter of 40 mm, a depth of40 mm and a parabolic sectional shape within the range of 20 mm to 40 mmfrom the upper end as shown in FIG. 12 in an Ar atmosphere atatmospheric pressure. The sample was held for 5 minutes after melting,and after the non-metallic inclusions were discharged, the sample wassolidified.

The surface of each sample after re-solidification was analyzed by thesurface electrolysis method of the present invention. For example, thesteel as the matrix was electrolyzed in a weight of about 0.5 g bysetting the sample to be melted into a 10% acetylacetone typeelectrolyte as an anode under the current density of 5 to 50 mA/cm². Thenon-metallic inclusions discharged on the sample surface were left inthe solution as the residue of electrolysis. After the electrolysis wascompleted, the non-metallic inclusions were collected as the residue onthe filter. Weighing and separation in accordance with the particle sizeor component analysis were carried out for this residue.

As the method of analyzing the non-metallic inclusions, the residue onthe filter was analyzed by the fluorescent X-ray analysis.Alternatively, after the filter containing the residue was heated andashed in a platinum crucible, it was fused by a fusing agent comprisingthe mixture of sodium carbonate, potassium carbonate and sodium borate,and after the fused product was heated and dissolved by using a dilutehydrochloric acid solution, it was analyzed by plasma emissionspectroscopic analysis or atomic absorption analysis.

An ultrasonic sieving method was employed as a method of measuring theparticle size of oxides. The residue on the filter was dispersed in amethanol solution or an ethanol solution by using an ultrasonic wave.This solution was poured onto a filter having a suitable mesh and wasfiltrated and classified by applying ultrasonic vibration. The particlesize distribution and the component composition of the non-metallicinclusions were determined by the weight of the residues and theirchemical analysis values on the respective filters.

As shown in FIG. 10, it was found out that the oxides which wereconcentrated on the surface by levitation-melting and wereelectrolytically extracted had substantially the same extractionfrequency in accordance with each particle diameter as that of theoxides extracted directly from the base metal by electrolysis which wasthe same as the sample used for levitation-melting. In other words,according to the evaluation method of the present invention, theinformation of the non-metallic inclusions of the base metal itselfcould be obtained without modification of the inclusions, etc., duringthe test. Therefore, the evaluation method of the present inventioncould drastically shorten the time necessary in the past for evaluatingthe quality of slabs.

FIG. 17 shows the results of the non-metallic inclusions contained inthe sample inside the tundish and the non-metallic inclusions containedin the sample, and their comparison result. As can be appreciated fromthe diagram, the evaluation method of the present invention couldevaluate economically, quickly and conveniently the non-metallicinclusions in the steel samples to find out, for example, that thequantity of the non-metallic inclusions was great in the tundish and wassmall in the slabs.

Example 5

The inventors of the present invention collected samples from a portionabout 30 mm below the skin of continuous cast slabs of a low carbonaluminum killed steel having a thickness of 250 mm, andlevitation-melted the samples by using a cold crucible having a maximuminner diameter of 30 mm in an Ar atmosphere at atmospheric pressure. Thelevitation-melted metal was then held for t seconds described later, andwas then solidified. Particles of non-metallic inclusions accumulatingon the surface of the solidified body could be observed by eye. Thesolidified body having the non-metallic inclusion particles accumulatingon the surface thereof was set as an anode to a 10% acetylacetone typeelectrolyte solution, and was electrolyzed to a weight of 0.5 g with theimpurity particles on the surface of the solidified body at a currentdensity of 5 to 50 mA/cm². Thereafter, the electrolyte solution wasfiltrated, and the residue on the filter was dispersed by the ultrasonicsieving method and was then poured onto a metallic filter having meshesof desired sizes to as to conduct filtration and classification byapplying ultrasonic vibration.

The present inventors conducted the experiment described above threetimes for a continuous cast slab of the same charge, that is, the casewhere the retention time was 60 seconds, 120 seconds and 180 seconds.The non-metallic inclusion particles were classified into the followingeight kinds in accordance with their sizes, that is, the first kind(exceeding 300 μm), the second kind (250 to 300 μm), the third kind (200to less than 250 μm), the fourth kind (150 to less than 200 μm), thefifth kind (100 to less than 150 μm), the sixth kind (50 to less than100 μm), the seventh kind (10 to less than 50 μm) and the eighth kind(less than 10 μm).

FIG. 18 shows the result of the experiment, and the ordinate representsthe number of the non-metallic inclusions per kg metal piece. As can beseen from the histogram of t=60" (solid line), the sizes of thenon-metallic inclusion particles occurring at the maximum frequency wereof five kinds and ranged from 100 to 150 μm when the retention time t ofthe levitation-molten metal was 60 seconds. As can be seen from thehistogram of t=120" (dotted line), on the other hand, the non-metallicinclusion particles were of seven kinds. As can be seen from the linegraph of t=180" (one-dot-chain line), the non-metallic inclusionparticles at the maximum frequency were of eight kinds when theretention time of the levitation-molten metal was 180 seconds.

As can be seen from FIG. 18, large non-metallic inclusion particles ofthe first to fifth kinds mostly accumulated on the surface of the moltenmetal at the retention time of 60 seconds, and their number hardlyincreased when the retention time was extended to 120 seconds or 180seconds. The non-metallic inclusion particles of the medium sizes of thesixth and seventh kinds did not sufficiently accumulate on the surfaceof the molten metal at the retention time of 60 seconds, but all of themgathered on the surface of the molten metal at the retention time of 120seconds. Further, their number hardly increased even when the retentiontime was extended to 180 seconds. The small non-metallic inclusionparticles of the eighth kind did not gather sufficiently at theretention time of 120 seconds, but almost all of them gathered on thesurface of the molten metal when the retention time was 180 seconds.

The present inventors collected the samples from the portions about 30mm beneath the skin of the same continuous cast slabs as those shown inFIG. 18 by using a cold crucible having a maximum inner diameter of 100mm in place of the crucible having the maximum inner diameter of 30 mmin FIG. 18, and conducted experiments in the same way. In this case, theretention time t of the levitation-melted metal was 55 seconds, 110seconds, 220 seconds and 330 seconds. The non-metallic inclusionparticles gathered on the surface of the levitation-melted metal wereprocessed and classified in the same way as in FIG. 18.

FIG. 19 shows the results of the experiments. As can be seen from theline graph of two-dot-chain lines representing the case of t=55",extremely large non-metallic inclusion particles of at least one kindgathered when the retention time of the levitation-melted metal was 55seconds, but the accumulation of smaller non-metallic inclusionparticles was not sufficient. The retention time t at which the fivekinds of non-metallic inclusion particles having the sizes of 100 to 150μm attained the maximum frequency was 60" in FIG. 18 but was 110" inFIG. 19. Similarly, the retention time at which the seven kinds of thenon-metallic inclusion particles having the sizes of 10 to 50 μmattained the maximum frequency was 120" in FIG. 18, but was 220" in FIG.19.

As described above, when the cold crucible having the different maximuminner diameter was used, it was not possible to accumulate thenon-metallic inclusion particles having the same size unless theretention time t of the levitation-melted metal was changed. Even whenthe inner diameter d of the crucible was changed, however, it waspossible to gather the non-metallic inclusion particles having the samesize if the ratio t/√ (30 d) of the maximum inner diameter of thecrucible to the retention time t of the levitation-melted metal remainedthe same as shown in FIGS. 18 and 19 when this ratio t/√ (30 d) wasused. In other words, when the ratio t/√ (30 d) was 2, five kindsattained the maximum frequency in both FIGS. 18 and 19 and when t/√ (30d) was 4, seven kinds attained the maximum frequency in both FIGS. 18and 19.

Therefore, in the present invention, when a crucible having a differentsize was used, the retention time of the levitation-melted metal wasadjusted by using t/√ (30 d). This adjustment made it possible tocorrectly grasp the non-metallic inclusion particles even when acrucible having different size was used, and made it also possible todirectly compare the results of the measurements of the non-metallicinclusion particles carried out by using crucibles having mutuallydifferent sizes.

According to an observation made by the inventors of the presentinvention, macroscopic non-metallic inclusion particles exceeding 300 μmobserved in the cold crucible method were not desirable for all kinds ofsteel materials, and it was always desired to determine them. As can beseen from the line graph of the two-dot-chain line in FIG. 19, thesemacroscopic non-metallic inclusion particles almost all accumulated whent/√ (30 d)=1. Therefore, t/√ (30 d) in the present invention was set toat least 1.

Though not shown in FIGS. 18 and 19, the observation of the presentinventors revealed that when t/√ (30 d) was at least 6, the quantity ofvery small non-metallic inclusion particles slightly increased. However,this slight increase of the very small non-metallic inclusion particlessaturated at t/√ (30 d) of 20. Therefore, the ratio t/√ (30 d) exceeding20 was not necessary, and t/√ (30 d) in the present invention waslimited to not greater than 20.

The present invention collected the samples from continuous cast slabsdifferent from those of FIG. 18 and conducted the experiments by using acold crucible having a maximum inner diameter of 30 mm by changing theretention time of the levitation-melted metal to 60 seconds, 120 secondsand 180 seconds. The impurity particles gathered on the surface of thelevitation-melted metal were processed and classified in the same way asin FIG. 18.

FIG. 20 shows the results of these experiments. Since the continuouscast slabs having a different charge from those shown in FIG. 18 wereused in FIG. 20, the numbers of non-metallic inclusion particles weredifferent from those in FIG. 18. However, the number of kinds of thesizes of the non-metallic inclusion particles occurring at the maximumfrequency when t/√ (30 d) was 2 was five kinds in the same way as inFIG. 18, the number of kinds of the non-metallic inclusion particles atthe maximum frequency was seven kinds in the same way as in FIG. 18 whent/√ (30 d) was 4, and the number of kinds of the non-metallic inclusionparticles at the maximum frequency was eight kinds in the same way as inFIG. 18 when t/√ (30 d) was 6. Large non-metallic inclusion particles ofthe first to fifth kinds mostly gathered on the surface of the moltenmetal at t/√ (30 d) of 2 in FIG. 20 in the same way as in FIG. 18, andtheir number hardly increased even when t/√ (30 d) was increased to 4 or6. The non-metallic inclusion particles having medium sizes of the sixthto seventh kinds did not sufficiently accumulate on the surface of themolten metal at t/√ (30 d) of 2, but mostly gathered on the surface ofthe molten metal at t/√ (30 d) of 4 and hardly increased thereafter evenwhen t/√ (30 d) was set to 6.

In other words, even when the charge of the continuous cast slabs to bemeasured was different, five kinds of non-metallic inclusion particlesappeared at the maximum frequency at t/√ (30 d) of 2, and seven kinds ofnon-metallic inclusion particles appeared at the maximum frequency att/√ (30 d) of 4. In the present invention, the cold crucible treatmentswere carried out by changing t/√ (30 d) to 2, 4 and 6 for the continuouscast slabs of the charge shown in FIG. 18, for example, and the resultthat the diameters L of the non-metallic inclusion particles occurringat the maximum frequency at each t/√ (30 d) were 5, 7 and 8 kinds, wasdetermined in advance.

When the number of kinds of the diameters was determined in advance asdescribed above, it became possible to estimate that the diameters L ofthe non-metallic inclusion particles occurring at the maximum frequencyin the case of FIG. 20 were of seven kinds, by carrying out the coldcrucible treatment by selecting the value 4 for t/√ (30 d) whencleanliness of the continuous cast slabs shown in FIG. 20 was evaluated.In this instance, the present invention measured the quantity N pcs/kgof the non-metallic inclusion particles having seven kinds of L.Alternatively, the quantities N₁, N₂, . . . , N₇ of the first to seventhkinds of non-metallic inclusion particles having L of at least 7 weremeasured.

For example, the continuous cast slabs were subjected to plastic workingto obtain steel products. In this instance, the non-metallic inclusionparticles invited the occurrence of defects such as scratches during theproduction process of the steel material and the steel products, andinvited also defects in quality such as the reduction of service life ofthe steel products. When means for plastic working was different andwhen the kind of steel products was different, the sizes of thenon-metallic inclusion particles that invited the occurrence of defectssuch as flaws and defects in quality changed, as well. In other words,there was the case where only the non-metallic inclusion particlesgreater than the seven kinds invited the occurrence of the defects butthe non-metallic inclusion particles smaller than the seven kinds didnot invite the defects, in accordance with the means for plastic workingand the kinds of the steel products.

In this case, it was not necessary to measure the occurring quantity ofthe non-metallic inclusion particles smaller than the seven kinds.Therefore, the cold crucible treatment was carried out by setting t/√(30 d) to 4, for example, and cleanliness of the metal could beevaluated by measuring the quantity N pcs/kg of the seven kinds ofnon-metallic inclusion particles. In this case, it was not necessary,either, to measure eight kinds of non-metallic inclusion particlesoccurring in greater quantities than the seven kinds, measurement ofcleanliness of the metal could be simplified and could be made easierthan the prior art method.

Example 6

The present inventors collected samples from continuous cast slabs oflow carbon aluminum killed steels having three different kinds ofcharges and a thickness of 250 mm, and each of the samples waslevitation-melted by using a cold crucible having a maximum innerdiameter of 30 mm in an Ar atmosphere at atmospheric pressure. Thelevitation-melted metal was retained for 120 seconds so as to gather thenon-metallic inclusions on the surface of the levitation-melted metal,and then the high frequency current applied to the coil of the coldcrucible was cut off. Ten and fifteen seconds later from cut-off of thehigh frequency current, the upper surface of the metal inside thecrucible was photographed by a CCD camera. In this instance, theoccupying portion of the non-metallic inclusions was formed in anisland-form on the upper surface of the metal, and the occupying portionof the non-metallic inclusions was photographed as the image of theislands due to the difference of luminance between the metal and theisland-like non-metallic inclusions. This image was subjected to imageprocessing so as to determine the areas of the occupying portions of thenon-metallic inclusions.

Each metal sample having the non-metallic inclusions accumulating on thesurface thereof inside the crucible was photographed by the CCD camera,and was taken out from the crucible after solidification. After theareas of the occupying portions of the island-like non-metallicinclusions were measured at normal temperature, the metal sample was setas an anode into a 10% acetyl-acetone type electrolyte solution, and themetal surface was electrolyzed to a weight of 0.5 g at a current densityof 5 to 50 mA/cm². After filtration, the weight of the non-metallicinclusions was measured.

The present inventors collected samples from continuous cast slabs oflow carbon aluminum killed steels having three different charges, andeach sample was levitation-melted by using a crucible having a maximuminner diameter of 100 mm in an Ar atmosphere at the atmosphericpressure. After the non-metallic inclusions were accumulated on thesurface of the levitation-melted metal by retaining thelevitation-melted metal for 400 seconds, the high frequency current tothe coil of the cold crucible was cut off. Ten to fifteen seconds afterthe cut-off of the high frequency current, the upper surface of themetal inside the crucible was photographed by a CCD camera.

The images so obtained were subjected to image processing in the sameway as when the crucible had the maximum inner diameter of 30 mm, andthe areas of the occupying portions of the island-like non-metallicinclusions were determined. The metal having the non-metallic inclusionsgathering on the surface thereof inside the crucible was subjected tomeasurement of the occupying areas of the island-like non-metallicinclusions in the same way as when the crucible had the maximum innerdiameter of 30 mm, and then the surface of the metal was electrolyzed toa weight of 1 g so as to measure the weight of the non-metallicinclusions.

FIG. 3 shows the results of these experiments.

                                      TABLE 3                                     __________________________________________________________________________             Occupying area of island-like                                          Maximum non-metallic Weight of non-                                           inner inclusions occupation area (mm.sup.2) metallic inclusions             diameter of            After obtained by                                        crucible Time from current cut-off solidification electrolysis (mg)         No.                                                                              (mm)  10 second (a)                                                                        15 second (b)                                                                        (c)   (d)                                              __________________________________________________________________________    1  30    201    201    176   25                                                 2 30 264 176 176 26                                                           3 30 327 138 113 14                                                           4 100 477 402 377 60                                                          5 100 590 465 465 70                                                          6 100 691 427 377 53                                                        __________________________________________________________________________

As can be seen from Table 3, the occupying area of the island-likenon-metallic inclusions was great when the time from cut-off of thecurrent was 10 seconds (a in Table 3) but dropped with the passage oftime and reached the smallest value after solidification (c in Table 3).FIG. 21 shows the occupying area of the island-like inclusions aftersolidification (c in Table 3) and the quantity of the non-metallicinclusions obtained by electrolysis (d in Table 3). As can be seen fromFIG. 21, the occupying area of the island-like inclusions aftersolidification had a close correlationship with the quantity of thenon-metallic inclusions obtained by electrolytic extraction. Therefore,evaluation of the quantity of the non-metallic inclusions could be madeby measuring the occupying area of the island-like non-metallicinclusions after solidification without conducting troublesomeelectrolytic extraction.

FIG. 22 is a diagram showing the relation between the occupying area ofthe island-like non-metallic inclusions after the passage of 15 secondsfrom cut-off of the current in Table 3 (d in Table 3) and the quantityof the non-metallic inclusions obtained by electrolysis (d in Table 3).Fluctuation was great in FIG. 22 in comparison with FIG. 21 but theoccupying area of the island-like non-metallic inclusions after thepassage of 15 seconds from cut-off of the current, too, had a closecorrelation with the quantity of the non-metallic inclusions obtained byelectrolytic extraction. Therefore, the occupying portion of theisland-like non-metallic inclusions formed on the upper surface of themetal inside the crucible after the passage of 15 seconds from cut-offof the current was photographed without awaiting the solidification ofthe sample, and the occupying area of the non-metallic inclusions couldbe measured by image-processing the image of the difference of luminancebetween the metal and the island-like non-metallic inclusions. In thisway, an evaluation could be made.

FIG. 23 is a diagram showing the relation between the occupying area ofthe island-like non-metallic inclusions after the passage of 10 secondsfrom cut-off of the current in Table 3 (a in Table 3) and the quantityof the non-metallic inclusions obtained by electrolysis (d in Table 3).As can be seen from FIG. 23, a high correlationship did not existbetween the occupying area of the island-like non-metallic inclusionsand the quantity of the non-metallic inclusions after the passage of 10seconds from cut-off of the current. Therefore, the occupying area ofthe island-like non-metallic inclusions after the passage of 10 secondsfrom cut-off of the current was not suitable as a scale for evaluatingthe quantity of the non-metallic inclusions. For these reasons, thepresent invention did not use the occupying area of the island-likenon-metallic inclusions after the passage of time less than 15 secondsfrom cut-off of the current for evaluating cleanness of the metal, butexclusively used the occupying area of the island-like non-metallicinclusions after the passage of at least 15 seconds from cut-off of thecurrent for evaluating cleanliness of the metal.

According to the observation of the present inventors, gathering of thenon-metallic inclusions to the surface of the levitation-melted metalwas not sufficient when the ratio t/√ (30 d) of the retention time t(second) of the levitation-melted metal in the cold crucible and themaximum inner diameter d (mm) of the crucible was less than 1. When t/√(30 d) was set to 1, large non-metallic inclusions having sizes of about300 μm accumulated on the surface of the molten metal. The non-metallicinclusions having the sizes of about 300 μm invited defects of the steelmaterial and the steel products during their production and use in manycases. Therefore, t/√ (30 d) was preferably set to at least 1 whenmanaging the non-metallic inclusions. When t/√ (30 d) was set to a valuegreater than 1, small non-metallic inclusions, too, gathered on thesurface of the levitation-melted metal with the increase of t/√ (30 d).Even when t/√ (30 d) was set to a value exceeding 20, however, thenon-metallic inclusions gathering on the surface of thelevitation-melted metal did not further increase. Therefore, theretention time t of the levitation-melted metal was preferably limitedto the range of 1≦t/√ (30 d)≦20.

FIGS. 7(a) and (b) are explanatory views useful for explaining themovement of the non-metallic inclusions gathering on the surface of thelevitation-melted metal. FIG. 7(a) is a schematic view when the highfrequency current was caused to flow through the coil to hold thelevitation-melted metal. In this case, a gentle stream 10 of the moltenmetal which rose at the center and flowed along the surface was formedinside the molten metal 4 that was levitated. Due to this stream 10 ofthe molten metal, the non-metallic inclusions 9 gathering on the surfaceof the molten metal were caused to flow towards the segments 1 and movedtowards them. When the high frequency current was cut off, the stream 10of this molten metal disappeared, too, and the non-metallic inclusionsthat had moved towards the segments 1 moved back to the center andformed portions of the island-like non-metallic inclusions as shown inFIG. 7(b). The reason why the occupying area of the island-likenon-metallic inclusions after the passage of 10 seconds from cut-off ofthe current is broadest in Table 3 (a in Table 3) was presumably becausethe non-metallic inclusions on the segment side 1 were moving towardsthe center when the time from the cut-off of the current was 10 seconds,gathering of the non-metallic inclusions was not yet sufficient and thenon-metallic inclusions were scattered on the surface of the moltenmetal 4.

Example 7

Metal samples were collected from two adjacent portions 30 mm beneaththe skin of continuous cast slabs of a low carbon aluminum killed steelshaving a thickness of 250 mm. One of the samples was levitation-meltedas a Comparative Example by a conventional apparatus having asingle-phase high frequency induction heating coil, and the other waslevitation-melted by an apparatus having three-phase A.C. inductionheating coil as an Example of the present invention. The supply of thecurrent was stopped after the passage of 10 seconds from melting, andeach metal sample was cooled to room temperature. The crucible used hada maximum diameter of 30 mm, and power supplied was 30 KVA in both casesand the high frequency was 100 KHz in both cases, too.

To evaluate the degree of accumulating of the non-metallic inclusions tothe top portion, the following evaluation was carried out.

The metal sample that was cooled to room temperature was set as an anodeinto a 10% acetylacetone type electrolyte solution, and the metalsurface was electrolyzed at a current density of 5 to 50 mA/cm². Ineither case, the metal surface on the top side of the levitation-meltedmetal was electrolyzed as the first step, and then the whole metalsurface was electrolyzed as the second step. The solution used for thiselectrolysis was filtrated, and the non-metallic inclusions werefractioned and their weight was measured.

The sum of the quantity of the non-metallic inclusions of the first stepand that of the second step in Example of the present invention wassubstantially equal to the sum of the quantities of the first and secondsteps in Comparative Examples. However, the quantity of the non-metallicinclusions of the first step in the Examples of the present inventionwas about 95% of the sum of the quantities of the non-metallicinclusions, whereas the quantity of the non-metallic inclusions of thefirst step was about 60% of the sum in the Comparative Examples. Inother words, since the non-metallic inclusions accumulated on the topportion side of the levitation-melted metal in the Examples of thepresent invention, substantially the whole quantity of the non-metallicinclusions were extracted by electrolysis of the first step. Therefore,the second step could be omitted, and the quantity of the non-metallicinclusions could be measured more quickly and more easily than in theprior art methods. On the other hand, since the non-metallic inclusionswere scattered on the entire surface in Comparative Example, thequantity of the non-metallic inclusions extracted by electrolysis of thefirst step was about 60%, and the second step was essentially necessary.

In other words, the evaluation method of cleanliness of the metalaccording to the present invention was the method which used the coldcrucible having the induction heating coil using the three-phase A.C.high frequency current, formed the upward stream on the surface of themolten metal which was levitation-melted, caused the non-metallicinclusions discharged to gather on the surface of the top portion of themolten metal, measured the non-metallic inclusions so accumulating onthe top portion of the molten metal, and evaluated cleanliness of themetal by this measured value.

The present inventors further collected samples from continuous castslabs of three low carbon aluminum killed steels having mutuallydifferent charges, and levitation-melted them by the apparatus havingthe three-phase A.C. induction heating coil of the present invention.After each sample was held under the levitation-melted state for about10 seconds, the supply of power was stopped, and the top portion of eachmetal sample during cooling was photographed by the CCD camera. Becauseluminance of the metal during cooling was different from that of thenon-metallic inclusions, an image wherein the island-like occupyingzones of the non-metallic inclusions were formed at the center could beobtained in each case. The area of the occupying zones of thenon-metallic inclusions was measured by processing the image soobtained.

Each metal sample was cooled to room temperature, and the metal surfacewas electrolyzed in the same way as described above. Thereafter, theweight of the non-metallic inclusions was measured. FIG. 24 is a diagramshowing the relation between the area of the island-like occupying zonesof the non-metallic inclusions and the quantity of the non-metallicinclusions obtained by the electrolytic method. As can be seen from FIG.24, an extremely close correlationship could be observed between them.When the quantity of the non-metallic inclusions was measured by theelectrolytic method, cooling, electrolysis, filtration, weighing, etc.,of the metal sample were necessary, the processing was complicated, andthe processing time was long. When the area of the island-like occupyingzones of the non-metallic inclusions was measured, on the other hand,the processing was extremely simple, and could be conducted within anextremely short time. Therefore, the present invention measured theisland-like occupying area of the non-metallic inclusions and evaluatedthe quantity of the non-metallic inclusions by this area. The method ofmeasuring the island-like occupying area of the non-metallic inclusionsprovided the highest accuracy, was easy to practice quickly, and wasextremely suitable when the non-metallic inclusions were used as aguideline for the production or use of the steel materials.

Example 8

Casting samples were collected from a molten steel of a low carbon steelinside a tundish during casting by a continuous casting machine, andrectangular samples having a weight of 100 g were cut out. Each samplewas then molten by using a cold crucible apparatus in an Ar atmosphereat the atmospheric pressure, was retained for 5 minutes aftersolidification and was solidified after inclusions were discharged.

The surface of each sample molten by the cold crucible was analyzed byfluorescent X-rays. Measurement was carried out at a primary X-rayintensity of 1 μA×50 kV and an irradiation time of 90 seconds. Existenceindexes of alumina, silicate, calcia, etc., were determined from thefluorescent X-ray intensity of Al, Si, Ca, etc.

Each sample melted in the cold crucible was fixed between sample holderpads of a sample rotating apparatus in such a manner as to freely rotatethe sample round the center axis, and while the sample was being rotatedat 6 rpm, the fluorescent X-ray analysis was conducted. The results weretabulated in Table 4. Table 4 shows also the results when the sampleunder the stationary state was measured while the direction of thesample was changed, without using the sample rotating apparatus, as aComparative Example.

                                      TABLE 4                                     __________________________________________________________________________                   Alumina index                                                                            Calcia index                                        Sample         This Comparative                                                                         This Comparative                                      No. Measurement plane Invention Example Invention Example                   __________________________________________________________________________    1    Measurement plane 1                                                                          4.0        0.003                                             2  6.3  0.006                                                                 3  13.1  0.007                                                                4  7.1  0.006                                                                  8.2 mean 7.6 0.005 mean 0.006                                               2 Measurement plane 1  10.1  0.007                                             2  21.0  0.002                                                                3  13.7  0.003                                                                4  4.9  0.002                                                                  11.7 mean 12.4 0.006 mean 0.006                                             3 Measurement plane 1  mean 18.4  0.328                                        2  16.8  0.750                                                                3  4.6  0.157                                                                 4  4.4  0.071                                                                  11.5 mean 11.0 0.339 mean 0.326                                             4 Measurement plane 1  33.2  0.677                                             2  20.2  0.210                                                                3  17.9  0.408                                                                4  23.1  0.682                                                                 25.4 mean 23.6 0.488 0.494                                                __________________________________________________________________________

As can be understood from this Comparative Example, significant varianceexisted in the distribution of the non-metallic inclusions depending onthe measurement surface, and the correct result could not be obtainedunless the entire periphery of the side surface of the sample wasmeasured. The result by the present invention substantially agreed withthe mean value of the measurement values of the four surfaces ofComparative Example, and this indicated that the present invention couldbe used as the index of the non-metallic inclusions. On the other hand,the evaluation time per sample according to the present invention wasthree minutes, and an evaluation speed of about 1/3 of that ofComparative Example could be accomplished. Even when the measurement ofthe four surfaces of Comparative Example was carried out by using theinterrupted rotation mode of the sample rotating apparatus of thepresent invention, the evaluation time could be reduced by about 40%.

INDUSTRIAL APPLICABILITY

As described above, the method of the present invention can analyze andevaluate quickly and economically the non-metallic inclusions in themetal while keeping good representativity and correlation with theproduct. Quick evaluation of the inclusions by the present method can beapplied as a management index of the steel making operation whenintermediate products are forwarded to subsequent steps to guaranteequality, or as an evaluation index when a new process is developed andintroduced.

We claim:
 1. An evaluation method for cleanliness of a metal piececomprising the steps of:levitation-melting said metal piece for apredetermined time by using levitation-melting means thereby formingmolten metal on a surface of said metal piece; discharging non-metallicinclusions contained in said metal piece to said molten metal formed onsaid surface of said metal piece by said levitation melting; solidifyingthe molten metal on the surface of said metal piece; directly analyzingby a fluorescent X-ray analysis method a curved and non-smooth surfaceof said metal piece after solidification by using an energy dispersiontype spectroscope so as to measure quantities of elements constitutingsaid non-metallic inclusions and identifying the quantities of saidnon-metallic inclusions.
 2. An evaluation method for cleanliness of ametal piece according to claim 1, further comprising:after saidsolidifying step and during said direct analyzing, rotating eitherintermittently or continuously said metal piece having the curved andnon-smooth surface around an axis connecting an uppermost point and alowermost point during melting as a center thereof; and said identifyingstep further includes identifying quantities of said non-metallicinclusions in accordance with kind and type.